1. Field of the Invention
This invention relates generally to floating screed pavers and specifically to a smoothness feedback system for such pavers.
2. Description of the Related Art
In the laying of asphalt pavement roadways and the like, it is widespread practice to employ so-called floating screed paving machines. These machines include a tractor-like main frame having an engine for propulsion and for material distributing functions. Typically, there is a material receiving hopper at the front of the paver arranged to receive hot asphalt material from a truck as the paving machine advances along the roadbed. Slat conveyors or the like are provided to convey the material from the hopper, at the front of the machine, toward the floating screed, at the back of the machine. Immediately in front of the screed, there is typically provided a distributing auger, which receives the raw asphalt material from the slat conveyor and conveys it laterally so as to distribute the material along the front edge of the screed. As the machine advances along the prepared roadbed, the raw asphalt material flows under the screed, which levels, smooths and compacts it to provide a continuous, level pavement mat.
In a typical floating screed asphalt paver, the screed is attached to a pair of forwardly extending tow arms which engage the paver frame at their forward extremities. These tow arms are also connected to the paver frame by hydraulic or other actuators arranged to adjust the vertical position of the tow arm extremities in relation to the paver frame. By effecting proper control over the position of the tow arm forward extremities, the screed is maintained in relation to a reference plane or a reference element substantially independent of the irregular vertical motions of the paver frame itself. Thus, it is possible to cause the floating screed to lay a pavement mat which is smooth and level in relation to the underlying base surface.
Effective control of the screed may be achieved by means of a suitable position sensing device, for example, which is carried by one or both of the tow arms or other forward projections of the screed and arranged for contact with a predetermined reference surface. When the position sensing device becomes either higher or lower than is indicated by the reference surface, as with changing loads upon paver frame and/or irregularities in the roadbed surface, the tow point is caused to be controllably raised or lowered relative to the paver frame to maintain a constant relationship between the position sensing device, called the grade sensor, and the reference. In many applications, grade control is provided at only one side of the machine. For controlling the other side, there typically may be provided a so-called slope control, which functions to maintain a constant relationship between screed ends at opposite sides, either on a level basis or with a predetermined transverse slope.
In conjunction with the type of tow point control mentioned in the preceding paragraph, it is important to provide an appropriate reference for the position sensing device.
It has proven advantageous to utilize a mobile reference beam, which is carried along with the paver as it moves over the roadway base surface. An arrangement of this type is described and claimed in U.S. Pat. No. 3,259,034 to Davin, which is assigned to the assignee hereof and is incorporated herein by reference. In the arrangement of the Davin patent, an elongated beam structure is provided with a substantial plurality (e.g., ten) of independent supports, advantageously arranged on individual springs. The arrangement is such that, as the reference beam is carried along the base surface, it is supported by the combined action of the multiple, yieldable supports. The individual supports are enabled to follow the minor deviations in base contour without significantly affecting the position of the reference beam as a whole, and the mobile reference beam thus provides a suitably accurate, averaged reference plane representing the grade to which the pavement mat is to be applied. A sensing device carried by forward projections of the screed engages the reference beam near its center, to enable the screed to be maintained in a predetermined relationship to the moving reference beam.
U.S. Pat. No. 3,846,035 to Davin, which is assigned to the assignee hereof and is incorporated herein by reference, disclosed a moving reference beam arrangement in connection with the laying of wide pavement mats, utilizing a combination of reference beams, one being towed ahead of the screed and auger, supported on the roadway base grade, and the other being towed behind the screed and auger, supported on the just-laid asphalt mat. To greatest advantage, the arrangement of the Davin '035 device includes a system of compound levers associated with the respective leading and trailing reference beams and arranged to derive a signal which is a function of the relationship of the screed and its tow arms to the respective reference beams. In effect, the reference beam arrangement is of greatly increased length and enables the laying of a mat of increased smoothness and accuracy in comparison to prior arrangements.
The trailing reference beam, which is towed by the paver frame behind the auger and screed is supported by one or more shoes or wheels. In the case of either the leading or trailing reference beams, or both, it is considered preferable to utilize a rigid beam with independently yieldably moveable supports.
In its most advantageous form, the system includes leading and trailing reference beams of rigid structure, independently supported by a plurality of yieldable supports. The leading reference beam is supported by a plurality of shoes or plates, while the trailing beam is supported by a pair of shoes. Elongated reference arms extend rearwardly from the leading beam and forwardly from the trailing beam and are pivotally connected to one of the screed tow arms, advantageously at a point forward of the screed itself but well behind the tow point. Thus, the reference arm pivot point is in a position to reflect deviations from the reference level of both the tow point and the screed itself.
After a pavement is laid by a machine similar to those described above, one of the most important measures of the quality of the newly paved road surface is smoothness, that is, the number and size of bumps and dips in the pavement. Smooth roads require less maintenance and help conserve fuel. They also provide for a more comfortable ride. Because of the importance of smooth roads, most contractors must adhere to strict specifications concerning the smoothness of the roads they construct. A road which does not meet the specifications may result in the forfeiture of part of the contract price or may require grinding or filling parts of the pavement, both of which are costly to the contractor. On the other hand, pavement which exceeds specifications for smoothness may result in bonus payments to the contractor. Thus, it is desirable to obtain smoothness data on a newly paved road to determine whether specifications are being met.
A number of devices have been used for measuring the smoothness of a road. The most common currently in use is the profilograph, which is an elongated beam or frame supported on several wheels. The beam establishes a datum from which deviations in the road surface can be measured. A sensing wheel rolls on the surface and moves vertically as it travel over bumps and dips in the road. Originally, profilographs were entirely mechanical devices which used a linkage to transmit the vertical movement of the sensing wheel to a pen which traced a plot of the road surface on a moving roll of paper. Such a plot is analyzed by laying a template with a "blanking band" over the plot. The blanking band defines a tolerance and blanks out minor aberrations.
Profilographs have advanced to the point where data from the sensing wheel is transmitted electrically and can be printed or stored in a computer for later analysis. Some computers provide the capability to automatically analyze the plot by applying an electronic blanking band.
Profilographs have proven to be useful in measuring the smoothness of a fresh asphalt mat, however, they are generally not used until several hours or even days after the mat has been laid and rolled. It is desirable to obtain smoothness data for a fresh mat immediately after it has been laid prior to rolling so that the mat can be worked while it is still plastic. Also, it is desirable to know if the paver or crew are not paving properly before an entire job is completed so that corrections can be made for the remainder of the job.
Since pavers are not equipped to directly sense the location of the screed relative to the reference, a position of the tow arm, forward of the screed, is typically used to sense screed position. Complete precision in controlling the screed is not possible because of inherent problems in mechanical linkages which are used. Thus, immediate smoothness data can help improve paver performance by immediately indicating problems with screed control which affect smoothness. For improved efficiency, it is preferred that the smoothness data is collected as an integral part of the paving operation.